At work, it's an open WiFi lan without encryption. At home, I have WEP
with a certain key. At my girlfriend's, it's WEP with a different key.

They all have different SSID's.

What I would like, of course, is to be able to configure my Debian
laptop so that, when I boot the laptop at any of these locations, the
laptop sees the SSID and automatically uses the right settings for that
location.

Seems simple enough to do. Windows does it. I just can't figure out
which config files to tweak in Debian.